1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas which can remove nitrogen oxides contained in an exhaust gas discharged, for example, from internal combustion engines of automobiles, chemical plants such as plants for producing nitric acid, etc. More specifically, the present invention relates to a catalyst for purifying an exhaust gas which can effectively remove NOx in an oxygen rich atmosphere, such as an exhaust gas from a lean burn engine, within a broad temperature region. The term "an oxygen rich atmosphere" used herein means an atmosphere having an oxygen content larger than that of a stoichiometrical air-fuel ratio.
2. Description of the Related Art
It is known that nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC), which are toxic substances contained in an exhaust gas discharged from internal combustion engines, are removed by, for example, using a three-way catalyst comprising platinum, rhodium, palladium, etc., supported on a carrier. In the case of an exhaust gas discharged from diesel engines, however, the three-way catalyst is not effective for removing nitrogen oxides because the exhaust gas contains a large amount of oxygen.
In recently developed gasoline engines, lean burn has become necessary for the purpose of lowering the fuel consumption and reducing the amount of exhausted carbon dioxide gas. Since, however, an exhaust gas from this lean burn gasoline engine comprises an oxygen rich atmosphere, the above-mentioned three-way catalyst was not effective.
In view of these circumstances, various proposals have been made on catalysts for purifying an exhaust gas from automobiles wherein the oxidation of carbon monoxide (CO) and hydrocarbons (HC) and the reduction of nitrogen oxides (NOx) are simultaneously effected. A copper-containing-zeolite catalyst comprising a zeolite ion-exchanged with copper is known in the art as a catalyst of this type for catalytic decomposition of NOx (see Japanese Unexamined Patent Publication (Kokai) No. 60-125250). Although this catalyst can remove NOx with high efficiency in an oxygen rich atmosphere, it has the problem that the region in which the catalyst exhibits catalytic activity is as narrow as 350.degree. to 400.degree. C.
A zeolite catalyst ion-exchanged with cobalt and an alkaline earth metal has been proposed as a catalyst for simultaneously removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen rich exhaust gas discharged from internal combustion engines of automobiles, the catalyst of which is less liable to suffer heat deterioration and has superior durability and high catalytic activity (see Japanese Unexamined Patent Publication (Kokai) No. 3-196842). This catalyst also has the problem that the region in which the catalyst exhibits catalytic activity is as narrow as 400.degree. to 450.degree. C. According to studies conducted by the present inventors, the problem concerning the region in which the catalyst exhibits catalytic activity is inherent in the supported metal components, that is, cobalt, alkaline earth metal and copper.